In a continuous inkjet printer the ink service module contains a working volume of printing ink and the make-up service module contains a volume of solvent make-up fluid. Make-up fluid is used to control the viscosity of the ink when the printer is in use, and to flush the print head, on start-up and shut-down, to ensure that the print head is clean.
Ink and solvent make-up are typically supplied to inkjet printing systems through the use of disposable bottles. An ink bottle may be mounted on an ink service module or reservoir so that ink can pass from the ink bottle into the ink reservoir. Each ink bottle, when full, contains a finite amount of ink, typically a pint or liter of ink. As the inkjet printing system is continually used, the ink within the ink bottle is depleted. When the ink bottle is fully depleted, a new ink bottle is mounted in place of the depleted ink bottle.
Hence the level of ink in the reservoir is maintained at a particular level.
The same applies with the solvent make-up bottle which is mounted on the make-up service module.
If the volumes of fluid in the ink service module or make-up service module are allowed to deplete there comes a point when the printer performance depreciates and, in some circumstances, the printer can be damaged. Therefore there is a need for fluid levels to monitored and action initiated in the following circumstances:                1. If the ink level is too high (‘High’), then an alert should be generated and the printer shut down to prevent ink overflow        2. As the ink level falls, an alert should be generated prompting the operator to replace the cartridge (‘Add Cartridge’).        3. If the level falls further to a predetermined minimum level (‘Low’) the printer should be shut down to prevent air ingress and motor damage.        4. It is also desirable to have an indication that the fluid level is in the normal operating range (‘OK’) i.e. neither too high nor at the point which the cartridge needs to be replaced.        
The existing continuous inkjet printer made by the applicant company includes an ink reservoir which is an open topped vessel on to which a manifold is fitted. The manifold is hydraulically connected to an ink management (distribution) block by flexible tubing. The manifold has an ink pick up tube
and a level sensing arrangement that is immersed in the ink. The level sensing arrangement is connected to the control system. When the reservoir is changed the manifold is removed from old reservoir and fitted in to the new one. During this operation ink can drip from the level sensor into the printer.
This can cause a mess and, depending on the nature of the ink, cause damage to the printer or its surroundings.
Capacitive level measurement is a technology that offers non-contact level measurement but problems arise when applying such technology to the measurement of ink levels in a continuous inkjet printer. Capacitive level measurement involves measuring the difference in dielectric constant between a fluid, such as ink or solvent make-up, and the environment above the fluid, typically air. One particular problem which arises when applying this technology to continuous inkjet printers is that these printers must operate using a variety of inks and the dielectric constants of these inks can vary significantly. This presents a drawback to an overriding requirement that ink type be changeable with minimal adjustment to the printer.
It is an object of this invention to provide a continuous inkjet printer, and/or one or more components therefor, and/or a method of monitoring fluid levels in a continuous inkjet printer which will go at least some way in addressing aforementioned problems; or which will at least provide a novel and useful choice.